Polyolefines and more specifically polypropylene and copolymers of propylene, have a relatively low crystallization rate compared to other crystalline polymers.
The crystallized polymers may, consequently, show a low degree of crystallization, large dimensions of the crystals or a non-homogeneous crystallization, which have a negative effect on the optical (transparency, translucence, etc..) mechanical (elasticity modulus, ultimate tensile stress, etc..) and rheological (moulding cycles and dimensional stability) properties of the injection moulded products.
To overcome these disadvantages, suitable nucleants are added to the above polymers.
The use of inorganic additives such as talc, silica, carbon-black and kaolin is already known. However these products have a limited nucleant effect.
The use of organic nucleants which favorably modify the crystallization process of crystalline polymers and copolymers of propylene, including mono or dibasic salts of aliphatic acids, such as sodium succinate, sodium glutarate, sodium caproate, etc.. is also known.
The use of the salts of alkaline metals or the aluminium salts of aryl carboxylic acids, such as aluminium benzoate, sodium or potassium benzoate, sodium beta-naphthoate, lithium benzoate, aluminium mono hydroxy -di-p.tertbutyl benzoate, etc.. is also known.
The use of organic nucleants, for example for isotactic polypropylene, is described in the U.S. Pat. Nos. 3,207,737, 3,367,926, 4,016,118, 4,532,280 and 4,585,817.
However the use of these organic nucleants has several disadvantages mainly due to the difficulty in being homogeneously dispersed in the polymer because of their low compatibility, their tendency to segregate in the dry mixing process and also their tendency to sublimate when the transformation temperature is high. Furthermore, some nucleants show the so-called "plate out" phenomenon, i.e. separation from the polymer and consequent deposit on the surface of the mould and final product.
Finally, it should be pointed out that all the above organic additives do not allow the heterogeneous nucleation of crystalline polymers with a controlled rheology modified by means of macroradicalic reactions at high transformation temperatures.
The European patent application 85,968 describes the use of nucleants made up of a combination of an aromatic monocarboxylic acid with a silicate or an alumina-silicate as a base, especially zeolites.
Because of their instability during the transformation of the polymer, these nucleants do not have a significant nucleant effect in polymers modified through a chemical restructurization process.
Nucleants made up of a product obtained by a reaction between an aryl carboxylic acid and a crystalline alumina-silicate of an alkaline or an alkaline earth metal, such as zeolitic tectosilicate according to the U.S. Pat. No. 4,518,731, are also unsuitable for this nucleant process on account of their thermal instability at the high temperatures required in the macroradicalic restructurization of polypropylene.